Witnesses reported artillery fire Friday around the state television building in the capital's sister city of Omdurman, just across the Nile.
The army announced it had brought reinforcements to the capital from other parts of Sudan to participate in "operations in the Khartoum area".
Sudan analyst Kholood Khair, the founder of Khartoum-based think tank Confluence Advisory. said the army was "expected to launch a massive offensive" to clear the paramilitaries from the streets of Khartoum.
For nearly seven weeks, deadly fighting between the regular army and the paramilitary Rapid Support Forces has gripped Khartoum and the flashpoint western region of Darfur despite repeated efforts to broker a humanitarian cease-fire.
Washington slapped sanctions on the warring parties Thursday, holding them both responsible for provoking "appalling" bloodshed.
The U.S. Treasury placed two major arms companies of the Sudanese Armed Forces, Defense Industries System and Sudan Master Technology, on its blacklist.
In parallel, it placed sanctions on gold miner Al Junaid Multi Activities Co and arms trader Tradive General Trading, two companies controlled by Rapid Support Forces (RSF) commander Mohamed Hamdan Dagalo and his family.
The State Department meanwhile imposed visa restrictions on both army and RSF officials, saying they were complicit in "undermining Sudan's democratic transition." It did not provide their names.
Analysts question the efficacy of sanctions on Sudan's warring sides, both of which amassed considerable wealth during the rule of longtime dictator Omar al-Bashir, whose government was subjected to decades of international sanctions before his overthrow in 2019.
So far neither side has managed to gain a decisive advantage. The regular army has air power and heavy weaponry, but analysts say the paramilitaries are more mobile and better suited for urban warfare.
The army will want to make "some military gains before committing to any future talks, in order to improve their bargaining position", said Khair.
Since fighting erupted on April 15, more than 1,800 people have been killed, according to the Armed Conflict Location and Event Data Project.
The U.N. says 1.2 million people have been displaced within Sudan and more than 425,000 have fled abroad.
Some 25 million people - more than half Sudan's population - are now in need of aid and protection, the U.N. says.
Aid corridors that had been promised as part of the abortive humanitarian truce never materialized and relief agencies say they have managed to deliver only a fraction of the needs.
Medics and relief agencies say 18 aid workers have been killed as widespread looting has gripped the main battlegrounds.
Aid agencies have reported "the theft of 115 vehicles and the looting of 57 warehouses and 55 offices," according to the mediators of the failed truce.
Later Friday, the U.N. Security Council was set to review its mission in Sudan, whose mandate expires on Saturday.